Method and arrangement for purifying gases fed to a fuel cell by removing operational unfavorable constituents

ABSTRACT

A method and an arrangement are provided for purifying gases which are fed to a fuel cell for operation. A filter system, which is designed to separate out particulates and polluting gases, is arranged in a feed passage for the respective gas.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and an arrangement for purifying thegases that are to be fed to a fuel cell for operation by removingconstituents which are unfavorable to the fuel cell operation.

On account of their high efficiency and the low levels or absence ofpollutant emissions, fuel cells are also used in electric vehicles. Byway of example, an electric vehicle is known which has a driving motor,a fuel cell and a fuel tank, a water store, an evaporator and areformer. The fuel tank contains methanol which, together with waterfrom the water store, is converted into the gaseous state in theevaporator and is then passed to the reformer, in which substantiallyhydrogen, carbon dioxide and carbon monoxide are formed with heat beingsupplied by a catalytic burner. The carbon monoxide can be oxidized withan oxidizing agent. The hydrogen-containing fuel gas from the reformeris fed to the fuel cell by way of a compressor; the fuel cell comprisesa fuel cell stack in which a multiplicity of individual fuel cellmodules are integrated. Humidified air is fed to the fuel cell by afurther compressor. Electrical energy for the electric driving motor isgenerated in the fuel cell from the hydrogen and from the oxygen of theair (DE 44 12 450 A1).

Membrane fuel cells, which in each case have a proton-conducting ionexchange membrane made from a polymer material, e.g. fluorinated resin,with a very good electrical conductivity in the moist state, are alsoused in electric vehicles. The membrane surface is covered with acatalyst. On one side, the electrolyte membrane is connected to agas-permeable anode, and on the other side, it is connected to agas-permeable cathode. A ribbed, gas-impermeable plate adjoins theanode; the cavities between the ribs of this plate serve to supply theoxidizing gas, e.g. air, with an oxygen content. A gas-impermeableribbed plate likewise adjoins the cathode, and its cavities between theribs are used to supply the gaseous fuel, e.g. the hydrogen-containinggas. When the fuel cell is operating, the electrolyte membrane ismoistened by the water of reaction and the humidity in the gasessupplied.

A fuel cell in a fuel cell system requires sufficiently purified gasesand/or gas mixtures, both with regard to the fuel gas and with regard tothe oxidizing gas, for it to operate if the original gas containsconstituents which have an adverse effect on the mode of operation ofthe fuel cell. In this context, the term fuel cell is to be understoodas meaning both an individual fuel cell module with the structure asdescribed above and a stack of fuel cell modules of this type, which canbe connected in parallel and/or in series. If the oxidizing gas used isair, which is drawn in from the atmosphere, it is generally necessary tocarry out a purification step. When using hydrogen, which is taken froma tank or is generated from a liquid fuel, it is often the case thatthere are no disruptive constituents in the gas, and consequently thereis no need for any purification.

WO 02/22234 A2, which forms the generic document, discloses an airfilter system for low-temperature catalytic processes for fuel cells. Inthis system, a very wide range of particles and gases/vapors arefiltered out of the incoming air. The filters can be divided intophysical or particulate filters and chemical filters. The filters areconnected in series in a housing. If the concentration of a pollutant inthe incoming air drops below a certain concentration, the chemicalfilter is spontaneously regenerated through desorption.

The invention is based on the problem of providing a method and anarrangement for eliminating contaminating substances from the gases foroperation of a fuel cell before the gases are fed into the fuel cell.

In a method of the type described above, the object is achieved,according to the invention, by the fact that: (a) the gas(es) are passedacross a filter system which is designed both to separate offparticulates and to remove constituents in gas and vapor form which havea damaging effect on operation of the fuel cells, and (b) the gas(es)are fed to the fuel cell on leaving the filter system. In this case, thegas(es) are passed across a filter system which can be regenerated andis monitored on the basis of criteria indicating a drop in the filteraction and that the regeneration should be carried out, with a messagebeing generated when these criteria are reached. The regeneration iscarried out when the fuel cell is inoperative. This makes it possible toavoid damage to the fuel cell in the event of spontaneous releases ofpollutants during regeneration.

The method according to the invention removes not only particulates,such as dust and carbon particles, but also further constituents of thegas, such as polluting gases, aerosols, organic substances, such asalgae, spores, bacteria and viruses, from the gas or gases, which arealso referred to below as reaction gases, although they may comprise amixture of gases, not all of which react in the fuel cell or contributeto the generation of electric power. The purification of the gas orgases makes it possible to lengthen the operating time or service lifeof the fuel cell.

In an arrangement of the type described above, the object is achieved,according to the invention, by virtue of the fact that a filter systemis arranged at a location in the gas-carrying passage for feeding thegas to be purified to the fuel cell, which filter system separates outboth particulates and constituents in gas or vapor form, which have adamaging effect on operation of the fuel cell. In this case, the gas(es)are passed across a filter system which can be regenerated and can bemonitored on the basis of criteria indicating a drop in the filteraction and the execution of the regeneration; a message can be generatedwhen these criteria are reached. The purification of the respectivereaction gas using the filter system prevents impurities from beingdeposited in the feed passages, in delivery devices and in the fuel cellitself, thereby gradually causing the function of the fuel cell todeteriorate, or prevents polluting gases from causing undesirablereactions in the fuel cell.

In an expedient embodiment, the filter system has a first filter forparticulates, downstream of which there is a second filter with asubstance for taking up and binding pollutants in gas or vapor form. Dryfilters made from plastic, glass fiber, paper with a high level ofdedusting can be used as the first filter and may, for example, have alabyrinth-like structure. The second filter includes, in particular,porous substances for taking up and physically or chemically bindinggases or vapors at the surface. Examples of substances of this typeinclude activated carbon or kieselguhr.

In another expedient embodiment, the filter system comprises a unit inwhich a dry filter for particulates and a substance for taking up andbinding gases or vapors at its surface are arranged together. Therefore,the filter system combines the functions of particulate separation andremoval of polluting gases. It is expedient for the particulate filterto include a substance for binding and/or separating off gases, whichsubstance is arranged on a material for separating out particulates oris self-supporting or forms a bulk bed.

In one preferred embodiment, the filter system is designed such that itcan be regenerated, it being possible for the regeneration to betriggered by an actuating element. This makes it possible to prevent theregeneration from being triggered spontaneously, e.g. when the fuel cellis operating, which can lead to high levels of pollutants being emittedand therefore to damage to the fuel cell. The regeneration of the firstfilter can be carried out, for example, using compressed air, whereasthe regeneration of the second filter can be effected by increasing thetemperature, since the adsorption is lower at a higher temperature thanat a lower temperature.

It is advantageous if the filter system is arranged in the gas-carryingpassage for the oxidizing gas upstream of the gas inlet of a compressor.The gas-carrying passage may be arranged inside or outside the fuel cellsystem to which the fuel cell belongs.

To establish the need for regeneration or filter maintenance with regardto particulate separation, in particular, the pressure differencebetween the pressure upstream and downstream of the filter system iscompared with a predeterminable limit value, with a message beinggenerated if the latter is exceeded.

With regard to the adsorption of pollutants, the need to regenerate orcarry out maintenance on the filter can be established using one or moregas or pollutant sensors downstream of the filter system, which are setto measure the pollutants which are to be separated off. The measuredvalues from the sensors are compared with a limit value in each case,with a message being generated if the latter is exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below on the basis of anexemplary embodiment illustrated in the drawings, from which furtherdetails, features and advantages will emerge. In the drawings:

FIG. 1 shows a fuel cell system having a filter system for removingcontaminating constituents contained in the gases to be fed to a fuelcell, in the form of a diagrammatic, partially sectional illustration;and

FIG. 2 shows another embodiment of a filter system for the fuel cellsystem illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

A fuel cell system 1, which, in a manner known per se, includes a fuelcell 2, for example of the type having an electrolyte membrane, andfurther components, which are not shown in the drawing, such as a fueltank, a water store, an evaporator, a reformer and a control unit, alsoincludes at least one apparatus 3 for sucking in and compressing a gas.This gas is, for example, air, the oxygen in which represents theoxidizing gas which in the fuel cell 2 reacts with the gaseous fuel togenerate electrical energy. The gaseous fuel is, for example, hydrogen.

The apparatus 3 includes a compressor 4, which sucks in and compressesthe air. The compressed air passes to the fuel cell 2 via passages (notshown in more detail) and, if appropriate, a control or metering valve.The compressor 4 used is, for example, a centrifugal compressor, whichis driven by an electric motor (not shown). On account of the relativelyhigh efficiency and low emission of pollutants, fuel cell systems arealso used in mobile apparatuses, such as motor vehicles. Small componentdimensions and low weights are of importance in these applications.

The apparatus 3 also includes a filter system 5, which is used to purifythe gas sucked in by the compressor 4. The filter system 5 is amulti-stage combination filter having a first filter 6, which is used toseparate particulates out of the intake air stream, and a second filter7, which is used to separate certain gases, aerosols and vapors out ofthe intake air stream. The first filter 6 has a plurality of filtersections. A first filter section 8, which is designed, in particular, asa porous intake passage, acts as a coarse filter for separating outparticulates, such as carbon particles or dust. A second filter section9 operates as a fine filter for particulate separation, for example, forremoving organic substances, such as pollen, and diesel carbonparticulates in the intake air. If air which is particularly free ofdust and particulates is to be generated, a third filter section 10 inthe form of an ultrafine filter is provided and is used to separate out,for example, bacteria, viruses and spores. The filter sections 8, 9, 10are, for example, dry filters. Whereas the filter section 8 may consistof a textile fabric, the filter sections 9 and 10 are composed oflabyrinth-like cells, which consist of plastic, glass fiber, paper ortextiles. A nonwoven design is also possible.

The second filter 7, the outlet opening of which is connected to thecompressor 4 through a gas-carrying passage 11, directly adjoins thefirst filter 6. The second filter 7 separates out gases which interferewith operation of the fuel cell 2 and are referred to below as pollutinggases. Adsorbents, such as activated carbon and kieselguhr, which cantake up and bind gases and vapors at their surface, can be used toseparate out the polluting gases. The air, which is sucked in by thecompressor 4, leaves the second filter 7 as a clean gas.

A sequence of the filter functions which differs from that describedabove is also possible. The sequence depends on thesubstances/particulates to be filtered out and the specific structure ofthe filter system 5. In addition to the separation or binding of thepolluting gases by adsorbents or other suitable substances, it is alsopossible for the polluting gases to be guided out of the air supplysystem.

Mechanical means, such as those referred to above in connection with theparticulates, or chemical, electrostatic or optical methods, which areknown per se, can be used for the filtering. A combination of thesemethods is also possible. In the case of optical filtering methods, forexample, UV or IR rays are used.

The combination of an electrostatic filter with ionization with filtersmade of fibrous substances in front of and/or behind it is alsoexpedient, so that the electrostatic filter is acted on uniformly andrelatively large particles, which have not been separated out in theelectrostatic filter or are entrained again, are removed from the airstream.

FIG. 2 diagrammatically depicts a sectional view through a filter system12, which includes both the elements for particulate separation and forthe physical and/or chemical binding of polluting gases, aerosols andvapors, in a single unit. A porous substance for binding or separatingoff gases is arranged on a carrier material 13 in the form of alabyrinth of cells of fibers. The porous substance on the carriermaterial 13 is represented by dots (not shown in more detail) in FIG. 2.The substance may be self-supporting or may also form a bulk bed. Thefilter system 12 is arranged between an intake passage 14 and thegas-carrying passage 11 leading to the compressor 4.

It is possible to use a filter system that can be regenerated. Thefilter system 12 diagrammatically represents one such system. Compressedair, which originates, for example, from a compressed air generator 15connected to the filter system 12, can be used for the regeneration inorder to remove the particulates which have been separated out. Gaseswhich have been bound by the substance can be released by heating thefilter system 12 in order to regenerate the filter system 12. Theregeneration requires additional devices, such as the blocking of thegas-carrying passage 11 and the opening of apertures for discharging theparticulates and gases released. These additional devices are not shownin FIG. 2.

The regeneration is expediently carried out while the fuel cell 2 isinoperative. To avoid spontaneous emissions of pollutants from thefilter system during regeneration, automatic regeneration is notprovided for. There is a control unit 17, which can be acted on by aninput element, e.g. button 16, and which determines the sequence ofregeneration. The control unit 17 is caused to initiate and carry outthe filter regeneration through actuation of the button 16.

It is also possible for some or all of the filter system to be designedsuch that it cannot be regenerated and for the parts or the filtersystem to be replaced during maintenance.

The accumulation of particulates causes the pressure difference acrossthe filter systems 5 and 12 to rise. The pressure difference can bemeasured using a device, e.g. manometer 18, and the measured value canbe compared with a predeterminable limit value; when this limit value isreached, a message is generated with the intention of indicating theneed to change the filter or carry out a regeneration operation. It isalso possible for the quantity of polluting gases taken up by anadsorbent to be checked for maintenance purposes by a sensor (notshown), which is suitable for determining the levels of pollutants thathave been adsorbed, being arranged in the gas-carrying passage 11. Ifthe sensor detects a predeterminable limit value for the pollutant(s), amessage is likewise generated. The measured values from the manometer 18and the at least one pollutant sensor are transmitted to an evaluationunit 19, in which they are compared with the predeterminable limitvalues.

The triggering of the filter regeneration can be effected not onlymechanically but also by electrical or optical means, in particular whenthe fuel cell system is inoperative.

The filtering of the reaction gases retains or removes both particulatesand gases, e.g. gritting salt, which is also dissolved and dispersed inthe atmospheric humidity. Gritting salt (deicing salt) is, for example,spread in winter and may be a constituent of the air which is sucked in.

The filtering of the reaction gases in accordance with the inventionprevents faults in the fuel cell system as a result of pollutants beingfed into the fuel cell, and thereby increases the service life oroperating time of the fuel cell.

An arrangement of the type described above is advantageously used in amobile apparatus, such as a vehicle, e.g. a motor vehicle, a locomotiveor a boat.

1. A method for purifying gases to be fed to a fuel cell for operation by removing constituents which are unfavorable to the operation of the fuel cell, the method comprising the acts of: passing the gas(es) across a filter system, which filter system both separates off particulates and removes constituents in gas or vapor form which have a damaging effect on the operation of the fuel cell; feeding the gas(es) to the fuel cell on leaving the filter system, wherein the filter system is regenerateable; monitoring the filter system on the basis of criteria indicating a drop in a filter action and a need for regeneration; generating a message when the criteria are reached; and preventing with a control unit regeneration when the fuel cell is operating.
 2. The method as claimed in claim 1, wherein a pressure difference in the filter system is monitored, and measured values from the monitoring act are transmitted to an evaluation unit and compared with a predeterminable limit value, with the message being generated when the predeterminable limit value is reached.
 3. The method as claimed in claim 1, wherein an actuating element causes a control unit to initiate and carry out the regeneration.
 4. The method as claimed in claim 2, wherein an actuating element causes a control unit to initiate and carry out the regeneration.
 5. An arrangement for purifying gases to be fed to a fuel cell for operation by removing constituents which are unfavorable to the operation of the fuel cell, the arrangement comprising: a filter system arranged at a location in a gas-carrying passage for feeding the respective gas to the fuel cell, which filter system separates out both particulates and constituents in gas or vapor form that have a damaging effect on operation of the fuel cell; wherein the filter system is regenerateable and is monitored on a basis of criteria indicating a drop in a filter action and the need for regeneration, with a message being generated when the criteria are reached; and a control unit preventing initiation of regeneration when the fuel cell is operating.
 6. The arrangement as claimed in claim 5, further comprising: a measuring device for measuring a pressure difference of the filter system, the measured values from the measuring device being transmitted to an evaluation unit and compared with a predeterminable limit value, with a message being generated when the predeterminable limit value is reached.
 7. The arrangement as claimed in claim 5, further comprising: at least one gas sensor for a polluting gas arranged downstream of the filter system, as seen in a flow direction of the gas, the measured values from the at least one gas sensor being transmitted to an evaluation unit and compared with a predeterminable limit value, with a message being generated when the predeterminable limit value is reached.
 8. The arrangement as claimed in claim 6, further comprising: at least one gas sensor for a polluting gas arranged downstream of the filter system, as seen in a flow direction of the gas, the measured values from the at least one gas sensor being transmitted to the evaluation unit and compared with a predeterminable limit value, with a message being generated when the predeterminable limit value is reached.
 9. The arrangement as claimed in claim 5, wherein the regeneration is triggerable by an actuating element.
 10. The arrangement as claimed in claim 6, wherein the regeneration is triggerable by an actuating element.
 11. The arrangement as claimed in claim 7, wherein the regeneration is triggerable by an actuating element.
 12. The arrangement as claimed in claim 9, further comprising: a control unit, which can be acted on by the actuating element in order to initiate and carry out the regeneration and which determines the sequence of regeneration.
 13. The arrangement as claimed in claim 10, wherein the control unit is actuatable by the actuating element in order to initiate and carry out the regeneration and to determine the sequence of regeneration.
 14. The arrangement as claimed in claim 11, wherein the control unit is actuatable by the actuating element in order to initiate and carry out the regeneration and to determine the sequence of regeneration.
 15. The arrangement as claimed in claim 5, wherein the filter system is arranged in the gas-carrying passage upstream of a gas inlet of a compressor.
 16. The arrangement as claimed in claim 12, wherein the filter system is arranged in the gas-carrying passage upstream of a gas inlet of a compressor.
 17. The arrangement as claimed in claim 5, wherein the filter system is composed of sections, which are connected in series and the filter function of which is matched to a constituent type to be filtered in the gas.
 18. The arrangement as claimed in claim 5, wherein a gas fed to the fuel cell is air, an oxygen content of which reacts with a fuel gas in the fuel cell.
 19. The arrangement as claimed in claim 5, wherein the filter system has a first filter for particulates, downstream of which there is a second filter with a substance for taking up and binding pollutants in gas or vapor form.
 20. The arrangement as claimed in claim 5, wherein the filter system comprises a unit in which a dry filter for particulates and a substance for taking up and binding pollutants in gas or vapor form are arranged together.
 21. The arrangement as claimed in claim 5, wherein said arrangement is part of a mobile device. 